1. Field of the Invention
This invention relates to a process for manufacturing organic silazane polymers which are suitably used as preceramic materials and also to a process for manufacturing ceramics from the organic silazane polymers.
2. Description of the Prior Art
Great interest has been currently shown in ceramics as materials which have good properties such as heat resistance, abrasion resistance, high-temperature strength and the like. However, because of the hardness and brittleness, ceramics are very difficult to process. For the manufacture of shaped ceramic articles, it is accordingly general to use a method which comprises molding a fine powder of ceramic material into a desired form such as by compression and sintering the molded article, or a precursor method in which an organic polymer, serving as a preceramic material, is melted or dissolved in a solvent, followed by processing the melted or dissolved polymer into a desired form and sintering it to render the polymer inorganic. The prominent feature of the precursor method resides in that ceramic products of such complex forms as will never be obtained in the sintering method for fine powder can be obtained, i.e. products of specific forms such as fibers or sheets can be manufactured.
Among ceramics, SiC and Si.sub.3 N.sub.4 have attracted generally considerable attention because of the good characteristic properties thereof at high temperatures, e.g. SiC has a high heat resistance and a high-temperature strength and Si.sub.3 N.sub.4 has a high thermal shock resistance and a high fracture toughness. Accordingly, there have been made various proposals on processes of producing SiC--Si.sub.3 N.sub.4 ceramics and also on processes of producing organic silicon precursors according to the precursor method as is particularly shown (1) to (5) below. However, these proposed processes have still problems set forth below.
(1) In U.S. Pat. No. 3,853,567, there is disclosed a process of obtaining SiC--Si.sub.3 N.sub.4 ceramics in which chlorosilanes and amines are reacted and subsequently heated at high temperatures to obtain carbosilazanes, followed by subjecting the carbosilazanes to spinning and infusibilization and then sintering at high temperatures of from 800.degree. to 2000.degree. C. However, this process requires high temperatures of 520.degree. to 650.degree. C. in order to obtain the carbosilazane, thus being very difficult to apply as an industrial process. In addition, the carbosilazanes are disadvantageous in that the yield of ceramic materials therefrom is as low as about 55%. As will be apparent from examples of this U.S. patent specification, the chlorosilanes used are only methyltrichlorosilane and dimethyldichlorosilane and the amine is methylamine alone.
(2) U.S. Pat. No. 4,097,294 describes conversion of various silicon-containing polymers into ceramic materials by pyrolysis. Only one silazane polymer is set forth in this patent and the ceramic yield is as low as 12% in a maximum. Although this U.S. patent specification describes that ceramic materials may be formed into fibers or thin films, the formation is merely suggested as possible. In fact, there is made little or no reference to moldability and processability of polymers which are considered to be most important in the precursor method.
(3) There is known production of silazane polymers, for example, by reaction between chlorodisilanes and disilazanes in U.S. Pat. No. 4,340,619, by reaction between chlorosilanes and disilazanes in U.S. Pat. No. 4,312,970, by reaction between chlorodisilanes and ammonia in U.S. Pat. No. 4,395,460, and by reaction between trichlorosilane and disilazanes in U.S. Pat. No. 4,543,344. Moreover, silazane polymers are prepared by addition of metal halides to chlorosilanes and disilazanes as disclosed in U.S. Pat. No. 4,535,007 and by addition of metal halides to chlorodisilanes and disilazanes as disclosed in U.S. Pat. No. 4,482,689. It is stated in these references that all the silazane polymers mentioned above may be converted to ceramic materials by pyrolysis. However, the ceramic yields of all the silazane polymers are, at most, 50 to 60 wt %. Similar to the U.S. Pat. No. 4,097,294, all the above references do not describe in detail moldability and processability of the polymers, which are most important in the precursor method. In particular, most references do not make mention of ceramic fibers in examples, or do not refer to strength of ceramic fibers in case where examples of ceramic fibers are shown. Only in U.S. Pat. No. 4,482,689, there is a description of strength, but ceramic fibers having such a low tensile strength as of 53 kg/mm.sup.2 or 63 kg/mm.sup.2 are obtained.
(4) In U.S. Pat. No. 4,482,669, there is described a process of preparing silazane polymers which comprises reacting ammonia with an organic silicon compound of the formula, ##STR1## to obtain an ammonolysis product and subjecting the product to condensation by dehydrogenation with alkali metal or alkaline earth metal hydrides to obtain silazane polymers. It is stated that the polymers obtained in this process can be controlled in property depending on the degree of condensation by deprotonation and may take various forms of from oils to solids having no definite melting points. However, when a polymer melt is molded or processed to prepare, for example, a continuous fiber by melt spinning, it is necessary that the polymer have a certain degree of polymerization and be thermally stable. In the above process, the polymer obtained will be in the form of a solid which has not a melting point unless the polymerization is stopped on its way. In order to obtain a fusible polymer, the reaction time, reaction temperature, amounts of a catalyst and a solvent have to be controlled precisely but such a control may be very difficult and may not be usually reproducible. The polymers obtained by the process are not thermally stable with the disadvantage that gel-like substances are formed. In view of the above two problems, this process may not be always considered to be suitable as an industrial process of manufacturing silazane polymers.
(5) Japanese Laid-open Patent Application No. 60-228489 describes a process of preparing a silazane polymer which comprises producing cyclic silazane from a compound of the formula, ##STR2## and monomethylamine, followed reacting the cyclic silazane with ammonia. In this patent application, it is stated that the polymer is suitable as a material for chemical deposition, but physical properties of the polymer are not described in detail. The ceramic yield is not indicated at all.
As will be apparent from the foregoing description, hitherto proposed silazane polymers, serving as preceramic materials, are not always beneficial for industrial production. In addition, these polymers were found to be poor with respect to moldability and processability into ceramic fibers and like with a poor ceramic yield. Ceramic products, e.g. ceramic fibers, obtained from the known preceramic polysilazane materials were found to have relatively poor physical properties such as strength, modulus of elasticity and the like.